The invention concerns a method for the disposal of nickel-cadmium or nickel-hydride cells.
Nickel-cadmium cells are widely used as backup elements (rechargeable batteries, accumulators) for supplying current, especially for portable electrical and electronic devices. In correspondingly large quantities such cells appear for disposal after they have become unusable or after they have been taken out of service. The main components of these cells are normally:
Housing (cover) of nickeled steel sheet, PA1 Grid electrodes of nickeled appertured steel sheet or interwoven wire, PA1 Sintered nickel powder as well as nickel hydroxide with the grid electrodes PA1 Cadmium as metallic powder and cadmium hydroxide, PA1 Isolating intermediate layers (dividers) of pressed synthetic wadding and paper, PA1 Plastic discs, possibly wood, PA1 Potassium hydroxide KOH as electrolyte (the portions between metallic Ni and Cd on one hand and their hydroxides vary according to the charge condition).
The orderly collection and disposal of such cells is required, above all because of their high content of highly toxic cadmium (around 20%), but also because of the nickel as well as the electrolyte; and on the other hand nickel and cadmium are also relatively valuable metals, for which a lively demand exists. Also, so called nickel-hydride cells have been known for some time and have increasingly come into use. In this type of rechargeable battery cadmium and cadmium hydroxide are replaced by special metal hydrides, for example titanium-nickel hydride, iron-titanium hydride, lanthanum-nickel hydride, etc., and have a high storing capacity for hydrogen. Although these cells do not include toxic cadmium, nevertheless they pose in a similar way the problem of disposal and the regaining of scarce valuable materials.
A state of the art disposal and retrieval by sorting of the valuable components of this kind of "waste" poses considerable difficulties, mainly because of the very compact laminated and partially sintered construction of the cells. Previously, essentially two methods were known and technically used:
According to a pyrolytic method the initially reduced in size waste material is heated to above 400.degree. C. in the presence of oxygen, whereby the Ni-hydroxide and Cd-hydroxide decompose and the organic components scorch. The residue is then heated to over 900.degree. C. with carbon, whereby the cadmium vaporizes and is retrieved by distillation. The remaining Ni and Fe can be delivered to ferro-nickel production. This method is uneconomical in that it has a high energy requirement and more than 30% slag is produced, which itself has to be disposed of. Especially, the control of the Cd vapor and the exhaust gas cleaning are expensive.
In the second method the entire waste mass is dissolved in mineral acid. The acid bath is then filtered, and the dissolved metals are individually acquired. The unavoidable decomposition of the entire metal content, including the iron, demands a large mass of acid and a corresponding extensive installation for the purification and maintenance of the acid bath.
According to an earlier, not previously published proposal for the processing of Ni--Cd batteries (CH Patent Application No. 3466/91-9 of Nov. 27, 1991) the waste mass is cooled with liquid nitrogen and embrittled and then reduced in size to 5 to 10 mm. The granules are subsequently washed and thereby freed of soluble electrolyte, then dried at 80.degree. to 90.degree. C. and then ground in a ball mill. Following this the mill product is separated into fractions largely by mechanical means and partially by renewed milling. According to this proposal most of the aforementioned difficult disadvantages can indeed be avoided, yet a satisfactory material separation--especially in the effective separation of the cadmium--is not achievable. It should be mentioned, that such difficulties are in part traceable back to the material processing by ball mills, which effects a partial compacting of the waste particles and makes considerably more difficult the subsequent mechanical material separation, especially by screens. Unfavorable moreover is the large requirement of wash water and the expense for its cleaning, as well as the energy and time expense for the drying of all of the materials.